1. Field of the Invention
This invention is concerned with a combination process for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures. In one aspect, the invention is concerned with a sequence of process steps for providing low ratio synthesis gas of less than 1 H.sub.2 /CO ratio and conversion thereof to premium hydrocarbon fuels including distillates and gasoline boiling range hydrocarbons. The process of the invention is particularly concerned with processing low ratio H.sub.2 /CO gases obtained from a coal gasification system of low cost and high fuel efficiency in a particular Fischer-Tropsch syngas conversion operation and catalytic upgrading of synthesis product to produce premium fuels.
2. Prior Art
Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in Encyclopedia of Chemical Technology, Edited by Kirt-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, N.Y., the contents of which are herein incorporated by reference.
It is well known that synthesis gas comprising carbon monoxide and hydrogen will undergo conversion to form reduction products of carbon monoxide, at temperatures in the range of 300.degree. F. to about 850.degree. F. and pressures in the range of 1 to 1000 atmospheres, over a wide variety of catalysts. The Fischer-Tropsch process, for example, which has been extensively studied, produces a wide range of hydrocarbons, waxy materials, oxygenates and some liquid materials which have been successfully used as relatively low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides or iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium with and without promoters.
The range of catalysts and catalyst modifications disclosed in the art encompasses an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen and provides considerable flexibility toward obtaining selected boiling range products. Nonetheless, in spite of this flexibility, it has not been possible heretofore to produce either olefin compositions comprising primarily internal double bond characteristics or aromatic hydrocarbons and boiling in the gasoline boiling range. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions", Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 4, pages 446-488, Interscience Publishers, New York, N.Y.
Compositions of iron, cobalt or nickel deposited in the inner adsorption regions of crystalline zeolites are described in U.S. Pat. No. 3,013,990. Attempts to convert synthesis gas over X-zeolite base exchanged with iron, cobalt and nickel are described in Erdoel and Kohle-Erdgas, Petrochemie; Brennstoff-Chemie, Volume 25, No. 4, pages 187-188, April 1972.
One particularly desirable catalyst used in the conversion of syngas has been potassium promoted iron, which has been used in combination with special types of zeolites, such as ZSM-5, in order to produce valuable hydrocarbons. Thus, for example, copending application Ser. No. 934,140 filed Aug. 16, 1978 is directed towards the conversion of syngas with potassium promoted iron in admixture with HZSM-5. Although the process of this copending application is indeed effective in producing products having a substantial quantity of aromatics, nevertheless there are disadvantages associated with said process, primarily in the regeneration aspect of the catalyst. It is known that when processes of this type are operated under conditions which favor the production of aromatics, there are also produced substantial amounts of coke which are deposited about the acid ZSM-5 catalyst. This requires that the catalyst be subjected to frequent regeneration, and due to the fact that the process of said copending application Ser. No. 934,140 involved a catalyst mixture containing an iron catalyst and a ZSM-5 catalyst, the extent and amount of regeneration were limited by the effect that the regeneration would have on the iron component. Thus, although HZSM-5 by itself exhibits a remarkable stability with regard to regeneration of the same by burning off carbon deposits, the same is not true with respect to a Fischer-Tropsch catalyst in general and iron promoted potassium in particular.
In U.S. Pat. No. 4,086,262 issued Apr. 25, 1978, there is disclosed a process for the conversion of synthesis gas using a single stage process wherein the catalyst is a mixture of an iron containing Fischer-Tropsch catalyst and a ZSM-5 type zeolite. One of the examples, however, is directed towards a two bed operation wherein syngas is contacted over a first bed containing an iron catalyst and the total product is thereafter contacted in a second bed containing a ZSM-5 type zeolite. The example resulted in poor aromatic production and excessive methane production.
In U.S. Pat. No. 4,046,830 there is disclosed a process wherein the total effluent from a Fischer-Tropsch operation is upgraded after removal of catalyst fines over a ZSM-5 type zeolite. Although the process of said patent is indeed a valuable one, it has been found that it can be significantly improved upon by operating within more selective process conditions.
The upgrading of a product of Fischer-Tropsch synthesis has also been disclosed in the following U.S. patents which have been considered in the preparation of this application. These U.S. Pat. Nos. are 4,041,096; 4,044,063; 4,044,064; 4,046,829; 4,046,831; 4,079,741; 4,052,477; 4,053,532 and 4,071,574.